Reduction in tandem loading



Dec. 4, 1962 P. H. ROSE 3,067,347

REDUCTION IN TANDEM LOADING Filed'uov. 12, 1959 m fi/f United States Patent Cfifice 3,067,347 REDUCTION IN TANDEM LOADING Peter H. Rose, Bedford, Mass, assignor to High Voltage Engineering Corporation, Burlington, Mass., :1 corporation of Massachusetts Filed Nov. 12, 1959, Ser. No. 852,288 3 Claims. (Cl. 313-63) This invention relates to apparatus for the acceleration of charged particles, and in particular to charged particle accelerators of the so-called tandem type wherein charged particles of one polarity are accelerated to a high voltage terminal within which the polarity of the charged particles is reversed, and the charged particles are then further accelerated away from the high voltage terminal. Patent No. 2,206,558 to Bennett, in US. Patent No. 2,213,140 to Kallmann, and in an article entitled Energy Doubling in DC Accelerators, by Alvarez, in The Review of Scientific Instruments, volume 22, page 705 (1951). Such a device usually includes a pair of acceleration tubes mutually aligned in end to end relationship and separated by the high voltage terminal. The high voltage terminal is raised to a positive potential and negative ions are injected into the grounded end of one of the evacuated acceleration tubes. The positive charge on the high voltage terminal attracts the negative ions towards it. The high voltage terminal is a hollow metallic structure so that its interior is shielded from external fields and is all at the potential of the high voltage terminal. After arriving at the high voltage terminal, the negative ions then coast through it at the speed which they have acquired by the previous acceleration. The negative ions have been formed by adding electrons to an otherwise neutral particle, and the process by which the negative ions are converted to positive ions is essentially a stripping process wherein not only the extra electrons but also at least one additional electron is stripped from the incoming negative ions. The device used for such stripping may be either a gas or a thin metal foil. In either case, low energy particles originate in the vicinity of the stripping device and a cloud of ions is formed near the exits of the stripper canal or near the stripper foil. These low energy ions are produced partly by ionization of residual gases, partly by field emission from metallic surfaces; the residual gases may be those that originate from the surfaces of organic materials or may be the gas which is emitted from the stripper canal. Those low energy charged particles which have a polarity opposite to that of the high voltage terminal have a tendency to oscillate about the high voltage terminal through the vacuum chamber traversing it Tandem accelerators are disclosed in US. I

and have suflicient kinetic energy to cause further ionization. The phenomenon thus becomes regenerative and results in power loss since the power required to produee and accelerate these spurious ions is of course derived from the high voltage generator. This phenomenon has been designated tandem loading.

In accordance with the invention, tandem loading is reduced by providing magnetic fields in the vicinity of the high voltage terminal which deflect the low energy charged particles so that they collide with some portion of the apparatus before they have acquired suflicient energy to cause the process to become regenerative. The magnetic fields may be produced by small permanent magnets since the strength of the magnetic field need only be of the order of 100 gauss. Moreover, the magnetic field may be either transverse or axial with respect to the beam trajectory. This is because it is only necessary that the particles be deflected slightly in order to eliminate them from the main ion stream.

The invention may best be understood from the follow- 3,067,347 Patented Dec. 4, 1962 ing detailed description thereof, having reference to the accompanying drawing in which FIG. 1 is a diagram illustrating a typical tandem accelerator with which the invention may be employed; and

FIG. 2 is a somewhat diagrammatic view in longitudinal central section showing the stripper device of the apparatus of FIG. 1, which is equipped with apparatus for reducing tandem loading in accordance with the invention. I Referring to the drawing and first to FIG. 1 thereof, the tandem accelerator therein shown comprises a high voltage terminal 1 which is raised to a high positive potential in the conventional manner by an endless travelling belt 2 of insulating material which transports electric charge between the high voltage terminal 1 and ground- Negative ions produced in a negative ion source 3 are injected into an acceleration tube 4 within which the negative ions are pulled towards the high voltage terminal 1'by the positive charge thereof. Within the high voltage terminal 1 there is provided a stripping device 5 which removes electrons from the incoming negative ions and converts them into positive ions. These positive ions emerge from the high voltage terminal 1 into a second acceleration tube 6 within which the positive ions are pushed away from the high voltage terminal 1 by the positive charge thereon.

The invention is not limited to use with any particular type of stripping device and merely for illustrative purposes there is shown in FIG. 2 a stripping device 5 which uses a gas to perform the stripping function. The strip ping device 5 includes a stripper tube 7 through which the charged particles pass and into which an appropriate gas is introduced via a gas supply tube 8, which may conveniently connect with the stripper tube 7 at the central point of the latter.

In accordance with the invention, magnets 9, 10 are provided in the vicinity of the point at which the charged particles enter and leave the high voltage terminal 1. For example, the magnets 9, 10 may be supported on the ends of the stripper tube 7. These magnets need only provide a magnetic field strength of gauss and may be arranged either so as to produce an axial magnetic field, as is produced by the magnets 9, or a transverse magnetic field, as is produced by the magnets 10. Since the principal incoming beam of negative ions is travelling at high velocity, this magnetic field has a negligible effect thereon. However, the low energy charged particles are easily deflected by the magnetic field sufficiently to remove them from a position where they would be able to re-enter the stripper tube 7. In this way, the regenerative oscillations characterized by movement of low energy charged particles back and forth through the stripper tube 7 are reduced.

The invention is not limited to any particular means for creating the necessary magnetic field. For example, the magnets 9, 10 shown in FIG. 2 may be replaced by equivalent electromagnetic coils; the magnets 9 may be replaced by an equivalent ring magnet; or one of the magnets 10 may be displaced so as to take the place of the magnets 9. As shown at 10, when transverse fields are used, it is preferable to provide at least one pair of oppositely oriented transverse fields to minimize deflection of the high-velocity ion beam.

The invention also comprehends biasing the stripper tube 7, or other stripping device, by a suitable bias voltage supply 11, so as to cause low-energy charged particles of polarity opposite to that of the bias voltage to be confined in the vicinity of the stripper tube 7, or other stripping device. Such a biasing arrangement may be used with or without the magnetic field of the invention.

Having thus described the principles of the invention, together with illustrative embodiments thereof, it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims.-

I claim:

1. In a tandem-type charged-particle accelerator including a hollow electrode at high positive potential, means for directing a beam of high-velocity negatively charged particles, which have been accelerated by said high positive potential, into said hollow electrode, and stripping rneanswithin said hollow electrode, whereby electrons are stripped from at least some ofthe charged particles traveling therethrough, the improvement which comprises means for producing a magnetic field in the path of said ueam within said hollow electrode but outside said stripping means, the strength of said magnetic field being insufficient to cause appreciable deflection of said high velocity charged particles.

2. Ina tandem-type charged-particle accelerator in cluding a hollow electrode at high positive potential, means for directing a beam of high-velocity negatively charged particles, which have been accelerated by said high positive potential, into said hollow electrode, and stripping means within said hollow electrode, whereby electrons are stripped from at least some of the charged particles traveling therethrough, the improvement which comprises means for producing a first magnetic field in the path of said beam after said beam enters said hollow electrode but before said beam enters said stripping device, and means for producing a second magnetic field in the path of said beam after said beam leaves said stripping device but before said beam leaves said hollow electrode, the strength of said magnetic field being insufiicient to cause appreciable deflection of said high velocity charged particles.

3. In a tandem-type charged-particle accelerator in: cluding a hollow electrode at high positive .potential, means for directing a beam of high-velocity negatively charged particles, which have been accelerated by said high positive potential, into said hollow electrode, and stripping means within said hollow electrode, whereby electrons are stripped from at least some of the charged particles traveling therethrough, the improvement which comprises means for biasing said stripping means with respect to said hollow electrode.

References Cited in the file of this patent UNITED STATES PATENTS 2,206,558 Bennett July 2, 1940 2,213,140 Kallmann- Aug. 27, 1940 2,258,149 Schutze Oct. 7, 1941 2,867,748 Van Atta et a1 Jan. 6, 1959 2,922,905 Van de Graaff Jan. 26, 1960 2,928,966 Neidigh Mar. 15, 1960 2,935,634 Lerbs May 3, 1960v 2,961,558 Luce Nov. 22, 1960 2,967,943 Gow -Jan. 10, 1961. 

